SI, Metric and Derived Unit

It is important that scientist use standards in both their work and communication. Standards are necessary in order to ensure everyone is on the same page and using the same units, definitions and words.

The SI and metric system are the standards used by scientists.

These standards allow scientists to make accurate and precise measurements that are reproducible and unchanging.

The metric system is a common system based on multiples of ten.

The SI system is just a modernized version of the metric system adopted in 1964 by the IUPAC and is based on fundamental units. All the other units are derived from the fundamentals.

People often misuse the terms accuracy and precision and it is very important the you understand what each term means.

Accuracy is how close the data matches the true value.

Precision is how well the data can be reproduced, that is repeated.

Believe it or not it is very possible to be precise, yet still be wrong. If your data is repeatable but not in line with the true value you are precise but not accurate. Many times a scientist will be wrong because the tools he is using are defective or imperfect. It is this very reason that many scientists work to improve scientific tools as much as the work on phenomena.

The SI System

The SI system (Systeme International d'Unites) has seven fundamental units and 22 derived units. It also allows for a few other units such as time but the cgs system is nit allowed.

Property	Unit	Symbol
length	meter	m
mass	kilogram	kg
time	second	s
temperature	Kelvin	K
electric current	Ampere	A
luminous intensity	candela	cd
amount of a substance	mole	mol

The Metric System

The metric system is a common measuring system based on powers of ten. Its fundamental units are the gram, meter and liter. In 1875 the worldwide scientific community accepted the metric system as the standard for science.

Property	Unit	Symbol
length	meter	m
mass	gram	g
volume	liter	L
time	second	s
temperature	Celsius	^oC

Prefixes

Both the metric and the SI systems are based on the decimal system (powers of ten) and make use of prefixes to indicate fractions and multiples of ten. The same prefixes are used in both systems.

Prefixes for Large
Prefix	symbol	meaning	value
giga	G	billion	10⁹
mega	M	million	10⁶
kilo	k	thousand	10³

Prefixes for Small
Prefix	symbol	meaning	value
deci	d	tenths	10^-1
centi	c	hundredths	10^-2
milli	m	thousandths	10^-3
micro	µ	millionths	10^-6
nano	n	billionths	10^-9

Conversion Rule:

large unit to small unit = multiply by the conversion

move the decimal to the right
small unit to large unit = divide by the conversion

move the decimal to the left

Home Page

Cycle Sheets

Videos

Handouts

Study Guides

What is Science

Physics Topics

◦ The Language
◦ Metric System
◦ Scientific Notation
◦ Significant Figures
◦ Unit Analysis
◦ Inertia
◦ Forces and Vectors
◦ Linear Motion
◦ Basic Trigonometry
◦ Energy

Physics Projects

◦ Mouse Trap Racer
◦ Catapult - Trebuchet
◦ Egg Drop Project
◦ Popsicle Stick Bridge